The invention relates to a method for forming a large-grain polycrystalline silicon film for use in thin film transistors, the film doped with a p-type dopant.
Thin film transistors are conventionally formed in silicon deposited over a substrate, often glass. Since the silicon is not epitaxially grown, it is not monocrystalline. Instead it is typically polycrystalline silicon, herein referred to as polysilicon.
Transistors formed in single-crystal silicon have no grain boundaries in the channel region. However, when the channel region of a transistor is formed in a polysilicon layer, grain boundaries in the polysilicon tend to decrease carrier mobility, degrading device performance. It is advantageous, therefore, to increase grain size, decreasing the number of grain boundaries.
Methods typically used to increase grain size include the introduction of catalysts, such as nickel or germanium, and laser annealing. Both of these methods have disadvantages.
One type of transistor that may be formed in polysilicon has a channel region lightly doped with a p-type dopant. This layer is conventionally formed by depositing an undoped film, then doping it using ion implantation. It would be advantageous to avoid this implantation step.
There is a need, therefore, to increase grain size in p-doped films to be used in thin film transistors without introducing metal catalysts or using laser annealing methods.